"He believes he's doing it all for the right reasons, and that's why people like Mandrake are so dangerous."
The Scot starred in action series 24, in the first Rambo film, and in sci-fi comedy series Red Dwarf.
McTavish said the intimacy and immediacy of Realiti was fascinating, and he admired director Jonathan King's ability to improvise. "He did it very cheaply, and very well. It's inspired me a little to explore doing something like that myself."
King said post-production finished a fortnight ago. He and friend Chad Taylor spent a few years developing the project. They didn't secure Film Commission funding but that didn't stop them.
The film-makers persuaded actors to get involved, and the shoot was tailored to their availability.
The result was a film unlike any most viewers would have seen before, King said.
"There are things about the film that are unconventional, because of the kind of film we wanted it to be. By making it independently, I guess we had the freedom to do that."
Realiti will have its premiere on July 31 at Wellington's Paramount Theatre.
The film also screens at the New Zealand International Film Festival in Auckland on August 1 and 2.
McTavish and New Zealand-born wife Gwen have decided to settle here.
"It was such a wonderful job from that point of view. It gave us a life opportunity that we seized with both hands."
McTavish returns to New Zealand in November.